In the production of a chemical pulp, chips of lignocellulose-containing material are cooked in an alkaline or acid aqueous solution. This cooking liquid contains inorganic pulping chemicals to improve the dissolution of lignin. The cooking is normally carried out at a temperature of above 100.degree. C. to reduce the residence time for the pulp produced. Therefore, the cooking is carried out in a pressure vessel known as a digester.
In the production of sulphate pulp, soda pulp and sulphite pulp with an alkali metal as a base, normally sodium, it is possible to recover the inorganic pulping chemicals in the spent liquor leaving the digester. It is vital both to the economy and the environment to recover these pulping chemicals to the largest possible extent. This is achieved in the pulping chemical recovery system, which essentially transfers the used inorganic pulping chemicals into a chemical state, where they can be re-used for cooking.
An essential part of the recovery system is the recovery boiler, where the spent liquor is burned. Normally, make-up chemicals are added to the spent liquor before the recovery boiler to make up for the chemicals lost during cooking and recovery. The spent liquor is sprayed into the lower part of the boiler, previously at a relatively low temperature to remove free water. Modern recovery boilers operate at a high temperature to reduce the content of sulphur in the flow gases leaving the boiler. Higher up in the boiler, gases and vapours of light hydrocarbons and decomposition products are volatilized. This is known as pyrolysis. Then, the pyrolysis products are burned after mixing with air or oxygen. The solid carbon-based residue which remains after complete pyrolysis of the organics is then heterogeneously burned. The solid particles formed are collected as a dust in precipitators at the top of the recovery boiler, to reduce the release of solid material to the surrounding atmosphere.
A substantial and increasing problem with the pulping chemical recovery system, is the presence of chloride and potassium in the spent liquor entering the recovery boiler. These elements tend to reduce the capacity of the recovery boiler to produce useful chemicals. Thus, chloride and potassium increase the stickiness of carryover deposits and dust particles to the recovery boiler tubes, which accelerate fouling and plugging in the upper part of the recovery boiler. Chloride also tends to increase the corrosion rate of superheater tubes.
Chloride and potassium are concentrated in the dust formed during the combustion of spent liquor in the recovery boiler. The dust is collected in dry-bottom or wet-bottom electrostatic precipitators. The dust mainly consists of sodium and potassium salts, where sulphate, carbonate and chloride are the dominant anions. The amount of dust corresponds to from about 5 up to about 15% by weight of the sodium entering the recovery boiler, which corresponds to from about 50 up to about 150 kg dust per tonne pulp, if the dust is calculated as sodium sulphate.
Today, normally all of the precipitator dust collected and withdrawn from the recovery boiler is recycled to the flow of spent liquor to be burned in the boiler. When the concentration of chloride or potassium is too high, a portion of the precipitator dust is withdrawn from the system and discharged or deposited.
The largest source of potassium is the wood, and the intake will depend on the wood source generally varying from about 0.5 to 5 kg per tonne pulp. The hardwood species usually contains larger amounts of potassium than softwood species. Besides, the content of chloride in the spent liquor can be very high for coastal mills, if the raw material consists of logs floated in seawater. As the environmental legislation becomes more stringent regarding pulp mill discharges to air and water, the degree of system closure increases. This means that even a small input of chloride and potassium becomes a severe problem, unless the content can be controlled by purging the system in some environmentally acceptable way.
Several methods have been proposed to overcome the problem with chloride and potassium build-up in pulping chemical recovery systems. The use of organic ion exchangers have been proposed as a unit operation for treatment of precipitator dust, but this has mainly been for softening purposes, e.g. to reduce the content of multivalent metals that would harm membranes in subsequent electrolysis of the precipitator dust. Chloride and potassium removal are preferably carried out in a common waste water treatment. Chloride can be removed efficiently by e.g. electrodialysis, while potassium still is difficult to remove efficiently electrochemically.
For instance, WO-A1-9404747 discloses a process, in which the content of chloride in a recovery system for pulping chemicals can be reduced. The process comprises collecting precipitator dust, dissolving the dust in water to produce an aqueous solution of precipitator dust, whereupon said aqueous solution is electrolysed in a cell for production of chlorine or hydrochloric acid in the anolyte. Use of ion exchange is suggested as a pretreatment before the electrolyses, chiefly to remove divalent ions such as Ca.sup.2+ and Mg.sup.2+.
Caron J. R. et al, "Metals management in a closed kraft mill bleach plant", Pulping Conference, TAPPI (1995), p. 1155-1160, have investigated metals removal from recycled chlorine dioxide bleach plant filtrate with ion exchange resins. The present invention overcomes many of the deficiencies of the prior art by providing an improved process by which the content of potassium ions in a recovery system for pulping chemicals can be reduced.